This invention relates to a method of purifying crude 4-fluorophenol containing phenol as impurity.
4-Fluorophenol is a compound known as an intermediate material for the preparation of some medicines and agricultural chemicals.
As described in J. Org. Chem., 26, 4641 (1961), 4-fluorophenol can be synthesized with comparatively good yields by alkaline hydrolysis of 4-bromofluorobenzene under superatmospheric pressure conditions. However, it is inevitable that 4-fluorophenol obtained by this method is contaminated with phenol which is formed as a by-product, and it is very difficult to isolate 4-fluorophenol from the crude product by reason of very close resemblances between this compound and phenol in their physical properties. That is, phenol cannot completely be separated from 4-fluorophenol even by superfractionation.
Another known method for the synthesis of 4-fluorophenol is the reaction of phenol with elemental fluorine in a suitable solvent such as aqueous hydrogen fluoride, but this fluorination reaction is very low in selectivity so that considerable amounts of 2-fluorophenol and 2,4-difluorophenol are formed together with 4-fluorophenol. Also in this case, 4-fluorophenol is likely to be contaminated with phenol remaining unreacted. It is also known that 4-fluorophenol can be obtained by hydrolysis of a diazonium salt of 4-aminophenol. However, in this case the yields are too low for industrial practice.
Under such circumstances, there is a keen demand for an efficient method of purifying 4-fluorophenol contaminated with phenol.